The invention relates to a method of generating a clock signal from a biphase modulated digital signal whose polarity is sampled in each symbol interval at a first and a second sampling instant at which instants the two polarity samples are compared with each other and a control signal in one sense is generated if there is equality of the polarity samples in response to which the two sampling instants are shifted relative to the biphase signal in a first direction, and if there is unequality of the polarity samples a control signal in the other sense is generated in response to which both sampling instants are shifted relative to the biphase signal in a second direction.
A method of this type is known from EP-A-0 162 505 (U.S. Pat. No. 4,709,378).
A symbol interval having length T of a biphase-modulated signal always has a phase transition halfway the interval, which phase transition is perceptible as a zero crossing if the maximum value of the biphase signal is positive and the minimum value is negative. At a distance of 1/4 T both to the left and right of this zero crossing the amplitude of the biphase signal presents a maximum (positive or negative) value and is a measure for the information content of the biphase signal in the symbol interval concerned. Hereinafter it will be assumed that the polarity of the maximum amplitude of the biphase signal is to be determined at a distance of 1/4 T to the left of the zero crossing. For the invention it does not matter at all whether this is effected to the left or right of the zero crossing.
In order to be in a position to establish the polarity of the maximum amplitude of the biphase signal in each symbol interval at a distance of T/4 to the left of the zero crossing, the biphase signal is to be sampled in the place of this maximum amplitude in each symbol interval. Since establishing the position of the maximum amplitude is much more difficult than establishing the position of the steep zero crossing, the biphase signal is sampled in each symbol interval at two instants which are mutually T/4 apart. If the second sampling instant coincides with the zero crossing, the first sampling instant will coincide with the maximum amplitude of which the polarity is to be determined.
By comparing the two polarity samples with each other and shifting these instants slightly to the right when they are equal and slightly to the left when they are unequal, after a number of symbol intervals the position of the second sampling instant will come in the neighbourhood of the zero crossing. The time required to this end is called the adjusting time. If the second sampling instant is then situated slightly to the left of the zero crossing, the two polarity samples are equal and the two sampling instants are shifted to the right so that in the next symbol interval the second sampling instant ends up slightly to the right of the zero crossing, whereas the first sampling instant is still situated to the left of the zero crossing in the neighbourhood of the maximum amplitude. The polarity samples taken at these instants will then be different so that both instants will be shifted to the left and the second sampling instant will again end up to the left of the zero crossing in the next symbol interval, and so on.
The two sampling instants continue to be shifted to the left and right while retaining a mutual distance of T/4 once the second sampling instant has been situated in the neighbourhood of the zero crossing. The first sampling instant then presents a jitter extending across the step size with which both sampling instants are shifted back and forth. For example, in digital echo cancelling systems the jitter is required to have a maximum of one thousandth part of the symbol interval. Such a small jitter could be realised by selecting the step size to be equally small, whereas the adjusting time then becomes inversely proportionally large. This is undesirable.
It is an object of the invention to provide a method according to which the first sampling instant does not present any more jitter after the adjusting time, without the adjusting time increasing.